Terrain Alterations

Following on from the introduction of Weather moves in Pokémon Gold & Silver, Pokémon X & Y has introduced a variety of new moves, essentially called Terrain moves. These moves will alter the terrain of the battlefield for 5 turns. During this time, various additional effects can be in place. These effects vary depending upon which terrain move was utilised, and whether or not the Pokémon is actually on the ground. If a Pokémon is Flying-type, or is off the ground, then it will not be affected by the special effects. Pokémon Sun & Moon pushed Terrain even further by adding a new terrain

You can now also extend Terrain by using the new hold item, the Terrain Extender which extends the terrain from 5 turns to 8 turns when activated. This works akin to the Damp Rock etc. with weather. Weather can also still be in effect at the same time as Terrain

As of Pokémon Sword & Shield, the Terrain can also be removed using the move Defog and the moves Steel Roller & Ice Spinner. In Ultra Sun & Ultra Moon it could be removed with the Z-Move Splintered Stormshards.

Electric Terrain
Activation Means: Use of the move Electric Terrain Use of the Max Move Max Lightning A Pokémon activates the ability Electric Surge or Hadron Engine The Wild Area areas of Axew's Eye, Bridge Field, Dappled Grove, Dusty Bowl, East Lake Axewell, Giant's Cap, Giant's Mirror, Giant's Seat, Hammerlocke Hills, Lake of Outrage, Motostoke Riverbank, North Lake Miloch, Rolling Fields, South Lake Miloch, Stony Wilderness, Watchtower Ruins & West Lake Axewell will sometimes have the weather Thunderstorm which activates Electric Terrain Effects: If the Pokémon is on the ground, it will wake up from Sleep, and be unable to fall asleep If the Pokémon is on the ground, the Pokémon's Electric-type move power will be boosted 30% as of Pokémon Sword & Shield. 50% before. The move Nature Power becomes Thunderbolt The move Rising Voltage increases in power by 50% The move Terrain Pulse becomes Electric-type and doubles power. The ability Surge Surfer activates, causing the Pokémon's Speed to double. The ability Hadron Engine causes the Pokémon's Special Attack to increase by 1 stage. Pokémon holding the Electric Seed item have their Defense boosted 1 stage. The item is then consumed. Pokémon with the Quark Drive Ability have their highest stat boosted by 30% (or 50% for Speed).
Grassy Terrain
Activation Means: Use of the move Grassy Terrain Use of the Max Move Max Overgrowth A Pokémon activates the ability Grassy Surge A Pokémon activates the ability Seed Sower by being hit by damage from an attack Effects: If the Pokémon is on the ground, it will recover 1/16th of its Hit Points each turn If the Pokémon is on the ground, its Grass-type moves will be boosted 30% as of Pokémon Sword & Shield. 50% before. Pokémon with the Grass Pelt ability gain a Defense boost of 50% The move Nature Power becomes Energy Ball The move Terrain Pulse becomes Grass-type and doubles power. The move Grassy Glide now has a Speed Priority of 1. Power of moves Earthquake, Bulldoze & Magnitude is reduced by 50% Pokémon holding the Grassy Seed item have their Defense boosted 1 stage. The item is then consumed.
Misty Terrain
Activation Means: Use of the move Misty Terrain Use of the Max Move Max Starfall A Pokémon activates the ability Misty Surge The Wild Area areas of Axew's Eye, Bridge Field, Dappled Grove, Dusty Bowl, East Lake Axewell, Giant's Cap, Giant's Mirror, Giant's Seat, Hammerlocke Hills, Lake of Outrage, Motostoke Riverbank, North Lake Miloch, Rolling Fields, South Lake Miloch, Stony Wilderness, Watchtower Ruins & West Lake Axewell will sometimes have the weather Fog which activates Misty Terrain Effects: If the Pokémon is on the ground, it will not be able to be affected by a status condition If the Pokémon is on the ground, and so is its target, their Dragon-type moves are reduced in power by 50% The move Nature Power becomes Moon Blast The move Terrain Pulse becomes Fairy-type and doubles power. The move Misty Explosion increases in power by 50% Pokémon holding the Misty Seed item have their Special Defense boosted 1 stage. The item is then consumed.
Psychic Terrain
Activation Means: Use of the move Psychic Terrain Completion of Mew's Special Z-Move, Genesis Supernova Use of the Max Move Max Mindstorm A Pokémon activates the ability Psychic Surge Effects: If the Pokémon is on the ground, it will not be affected by moves with a Speed Priority of 1 or higher. This does not affect moves with increased priority that the user uses on itself or moves that effect the field The move Nature Power becomes Psychic The move Terrain Pulse becomes Psychic-type and doubles power. The move Expanding Force increases in power by 50% and damages all opponents. If the Pokémon is on the ground, the Pokémon's Psychic-type move power will be boosted 30% as of Pokémon Sword & Shield. 50% before. Pokémon holding the Psychic Seed item have their Special Defense boosted 1 stage. The item is then consumed.